The scope of this article is to determine patients' knowledge about the medication they take. For this purpose, a cross-sectional, observational and descriptive study was conducted. Knowledge was measured by a valid and reliable questionnaire (CPM-PT-PT), given to the patients attending community pharmacies participating in the study, who had prescriptions for one or more drugs in the Lisbon Metropolitan Area. Knowledge was assessed in four dimensions: therapeutic objective, process of use, safety and maintenance of the medications that the patient takes. Thirty-five pharmacies participated, and 633 valid patients were obtained. Fully 82.5% (95% CI: 79.3% -85.3%) were uninformed about the nature of the drug they use. In all items, there was a high percentage of patients with incorrect knowledge, with emphasis on precautions (44.7%). The dimension that the patients were least aware of was "drug safety" (1.9%). Eight out of 10 patients in the population do not know what drug they use. The highest lack of correct information was with respect to the "safety" of the medication.